1. Contribution from the Department of Organic Chemistry, Umeå University, SE-901 87 Umeå, Sweden, School of Dentistry, University of Copenhagen, Nörre Allé 20, DK-2200 Copenhagen N, Denmark, and Institute of Molecular Pathology and Immunology, University of Porto, IPATIMUP, Rua Dr. Roberto Frias s/n, 4200 Porto, Portugal